Also known as Freedom Day, it commemorates the announcement of the abolition of slavery in Texas.
The day now more widely represents the emancipation of enslaved African-Americans across the whole of the south.
Here's everything you need to know about this significant day:
What is the history behind it?
This was more than two years after Abraham Lincoln's Emancipation Proclamation, which had legally freed slaves but the conflict had prevented it becoming a reality for them.
Why was there a delay?
Although the proclamation was signed in 1862, the Civil War raged on for another three years.
Federal troops arrived on 19 June 1865, imposing order and marking the end of the Civil War in Texas.
With the arrival of Union soldiers, led by General Gordon Granger, the momentous news finally came that slavery was legally abolished.
It was a commonly held belief that news of abolition was deliberately withheld so plantation workers could keep using slave labour for as long as possible.
Local legend even has it that a messenger on his way to Galveston to deliver news of the proclamation was murdered, preventing the information from getting through.
When did it become an official day?
Although the date had always been recognised, Juneteenth didn't become an official state holiday in Texas until 1980.
In memory of the historic event, 19 June is now a state holiday or a day of observance in most states. Not all though. Hawaii, Montana, New Hampshire, North Dakota and South Dakota do not recognise it.
Why is it called 'Juneteenth'?
The word is a portmanteau of 'June' and 'nineteenth'.
How is it celebrated?
It is one of the oldest celebrations commemorating the end of slavery and the day has been marked in different ways, with parades, church services, and cook-outs.
In the early days of celebration, there were accounts of former slaves throwing the ragged clothes they had worked in into rivers.
Juneteenth 2018 will see parades around the country, from New York to Texas to Philadelphia, with festivals big and small. Thousands of people are expected to take to the streets for walking parades in honour of the day.
Many of them will have buffets and barbecues of street food, one of the most popular ways to show the togetherness of the occasion.